1. Field of the Invention
The invention relates to a hydrostatic drive system comprising a demand-regulated pump and a plurality of consumers which are connected thereto and which can each be actuated by means of a directional control valve which performs a throttling function in intermediate positions. For load-independent allocation of the delivered flow in the case of simultaneously actuated consumers, each directional control valve is assigned a pressure compensator which can be controlled directly or indirectly by a signal difference formed from a load pressure signal and a delivered pressure signal, where the load pressure signal is derived from the highest of the load pressures occurring downstream of the directional control valves and the delivered pressure signal is derived from the pressure upstream of the directional control valves.
2. Description of the Prior Art
in such drive systems, the degree of opening of the directional control valves, independently of the load pressure of the consumers, determines the volume flow of pressure medium which is supplied to the consumers, and thus the speed of movement of the consumers. The load-dependent delivered flow allocation is effected here by pressure compensators, which are also known as load compensators and which can be arranged upstream or downstream of the directional control valves. It is also possible to integrate the pressure compensators into the directional control valves. However, control of the driving force and the driving moment of the consumers is not possible with such drive systems.
To ensure that the pressure medium is supplied to a consumer at a specified pressure, in DE-OS 31 46 561, which is not of the type defined in the introduction, it has been proposed that in the case of a specially designed directional control valve, which does not comprise an assigned pressure compensator for load-independent delivered flow distribution, the adjusting force which determines the degree of opening of the directional control valve be opposed by a force derived from the load pressure of the consumer.
In such a drive system, which can be used both for translational and for rotational consumers, the pressure medium is therefore supplied to the consumer at a specified pressure, so that the consumer is moved with a correspondingly predetermined force and a predetermined torque. The force (or torque) acting upon the consumer here is substantially continuously maintained constant; in the event of a reduction in the consumer load, and thus an initially falling load pressure, because of the uniform adjusting force across the directional control valve, its through-flow opening enlarges until, due to the now increased volume flow conveyed to the consumer, the load pressure returns to the original level and the force difference originally set is again attained across the directional control valve. In such a drive system, in addition to the aforementioned disadvantage of the specially designed directional control valve, load-independent actuation of the consumer, which is desirable under certain operating conditions, is also no longer possible. Furthermore, the force and torque limitation is not adjustable to different values.